The present invention is directed toward the field of power conversion systems. In particular, the invention is directed to a four quadrant power conversion topology which is especially suited for use with remotely powered devices having non-linear reactive loads.
Remotely powered devices are devices that are provided power from a power source located some distance away through the use of power transmission wires. One type of remotely powered electronic device is known as an optical network unit ("ONU"). An ONU functions within a system known as a Fiber-In-The-Loop ("FITL") system.
An ONU is a device within a FITL system that is used as an interface between fiber optic telecommunication lines and traditional wires used to provide telecommunication services such as cable television and telephonic services to homes or other buildings. The ONU has a power supply that typically includes: (i) input protection and filter circuitry; (ii) energy storage circuitry, (iii) input voltage monitors and threshold circuitry, (iv) D.C. to D.C. power converters; (v) ringing generators; and (vi) alarm and digital interface circuitry.
A FITL system includes a host digital terminal ("HDT"), which is connected to a central switching office via fiber optic lines, and a plurality of ONUs, which are connected to the HDT via fiber optic lines. The HDT provides telecommunication service access for the connected ONUs and power transmission wires for delivering power from the HDT, which has access to AC power, to the ONUs, which do not have access to AC line power. The power source within the FITL system is typically a 140V power source with a source resistance of 10 to 200 Ohms. At the remotely located ONU, the input voltage delivered by the power transmission wires is generally in the range of 70V to 140V, and can vary dynamically depending on the load on the system.
The power transmission wires used to supply power to an ONU are typically thin telephone wires. Because of the resistance in the thin telephone wires, the peak deliverable power to the ONU is extremely limited. To ensure that the power transmission wires are capable of delivering sufficient power, it is desirable to maximize the power conversion efficiency.
The ONU, like many other remotely powered devices, includes systems that are only intermittently active, but require substantial power when active. An exemplary system is the ringing generator within the ONU. The ringing generator must generate a 5 to 10 Watt low frequency alerting signal for ringing telephone sets that are connected to the ONU. To minimize the ONU's peak power requirement, the ringing signal must be generated in a power efficient manner.
The ringing waveform is typically a low frequency sinewave, in the 16.5 hz to 50 hz range. The electromechanical (ringer) load is nonlinear and reactive. Thus, the load current waveform is often not the same as the voltage waveform. Specifically, the current waveform may have zero-crossings at times different from the voltage waveform's zero-crossings. Moreover, the instantaneous current polarity may be independent of the instantaneous voltage polarity. Consequently, the ringing generator must be able to generate a bipolar sinewave voltage output while accommodating load current in either polarity.
This characteristic is commonly referred to as Four-Quadrant ("4Q") output capability. Within each cycle of the sinusoidal ringing waveform, the instantaneous power alternates in direction. The instantaneous power flow is toward the load when the output voltage and current have the same polarity. The power flow is toward the source (ringing generator) when the output voltage and current are of opposite polarity.
Previous generations of ringing generators provide 4Q output capability by using linear amplifiers or buck converters as output stages. Both of these output stages require a steady +/-100V supply voltages that are derived from the raw power source through power converters.
The theoretical maximum efficiency of a linear amplifier for a sinewave output is 2/Pi=63.7%. Taking into account the efficiency of supply converters, the overall efficiency falls somewhere between 50% and 60%.
The buck converter output stage can be designed to convert power in both directions and is thus more efficient than a linear power amplifier. But the double power conversion, from raw power to the regulated power supply rails for the buck converter and then the power conversion in the buck converter itself, incurs double conversion penalties. The power efficiency of such an arrangement typically falls between 60% to 76%.
Further, because the power transmission wires can be up to 6000 ft. long, the input power may have considerable common-mode components due to induction from the power wires. As a result, the input and output circuits must have galvanic isolation for typically 1500 V.
Therefore, there remains a need in this art for a 4Q power conversion topology with increased power efficiency. There remains a more particular need for a 4Q power conversion topology that has high galvanic isolation. Further, there remains a need in this art for a 4Q power conversion topology that can be implemented in remotely powered devices and can limit the remotely powered device's peak power demands.